Fluorescence detection in biological and diagnostic devices is the most frequently used technique to measure and quantify the presence of biological entities, like proteins, nucleic acids and cells, in a sample. Generally this is done by selectively binding the target entities to a substrate surface by specific capture molecules. The targets may, for example, be labeled by a fluorescent molecule and the presence of the target is identified by measuring fluorescence on the surface against a background of fluorescence from the surrounding, e.g. bulk fluid, bulk substrate, etc. Depending on the analytical challenge this detection must be achieved at high spatial resolution and with high sensitivity, even down to single molecule level and over a large surface area. Real time observation requires surface selective detection. Nano-photonic structures comprising waveguides or apertures allow such surface specific detection by creation of an evanescent field between the structures, made of a electrically conductive material. They therefore constitute an advancement of fluorescence based detection, which is currently in biological and diagnostic devices the most frequently used technique to measure and quantify the presence of biological entities.
Typically, such nano-structures which need to have a high aspect ratio are made of aluminium, since anisotropic etching of Al allows the fabrication of high aspect ratio features at economic conditions. Alternative materials include gold and other electrically conductive materials.
Unfortunately, many biological reactions and assays require specific buffer systems comprising, inter alia, high salt concentrations or other chemicals which tend to react with the surface of the nano-structures and lead to their degradation. In particular, aluminium surfaces are rather vulnerable at high pH. Due to the small size of the nano-structures of below 1 μm and the tendency of the preferred material to degenerate in required environments, there is a need for an effective protection of the nano-structures. According to US 2010/0252751 dielectric material such as SiO2 or Si3N4, which is applied isotropically, can be used as barrier coating material for nano-structures. The coating material needs to be conformal, i.e. has to protect the nano-structure from all sides and should lead to a pinhole free coverage. At the same time the coating should not be too thick in order to allow for access of the target molecules to the evanescence field. Si-based coatings as described in US 2010/0252751 were, however, found to provide no protection against buffer corrosion at the required thinness.
In consequence, there is a need for the development of an improved nano-structure barrier coating, which provides efficient protection against buffer corrosion during bioassays and allows for proficient evanescence field imaging.